In recent years, following the high integration of semiconductor devices, wirings of circuits have been miniaturized, and the distance between the wirings has also been reduced. Therefore, it is necessary to flatten the surface of a semiconductor wafer, which is a polishing target, and as one of the flattening methods, a semiconductor wafer is polished by using a polishing apparatus.
Conventionally, the polishing apparatus includes a polishing table for holding a polishing pad for polishing the polishing target, and a top ring holding the polishing target to be pressed onto the polishing pad. The polishing table and the top ring are rotationally driven by driving units (for example, motors), respectively. While a liquid (slurry) containing abrasive powder is poured onto the polishing pad, the polishing target held on the top ring is pressed thereonto, so that the polishing target is polished.
In the polishing apparatus, if the polishing target is insufficiently polished, there arises a concern that circuits are not insulated from each other, which causes short circuit with each other. Moreover, in the case of excessive polishing, the cross-sectional area of the wiring decreases to cause such problems that the resistance value increases, or the wiring itself is completely removed and thereby the circuit itself is not formed. Due to this, the polishing apparatus is required to perform optimum detection of the polishing end point.
As a polishing end point detection method, there is known a method of detecting a change in polishing friction force at the time when a different material begins to be polished with progress of the polishing (Japanese Patent Laid-Open No. 10-202523). A semiconductor wafer which is a polishing target has a layered structure formed of different materials including a semiconductor, a conductor and an insulator. The different material layers have different coefficients of friction therebetween. Therefore, in this method, the change in polishing friction force arising from a different material layer beginning to be polished is detected. According to the method, the time when the polishing reaches the different material layer is the end point of the polishing.
Moreover, the polishing apparatus can detect the polishing end point by detecting a change in polishing friction force at the time when the polishing surface of the polishing target changes from being not flat to being flat.
Herein, the polishing friction force arising in polishing the polishing target can be seen as driving load of the driving unit. For example, in the case where the driving unit is an electric motor, the driving load (torque) can be measured as a current flowing through the motor. Due to this, the motor current is detected by a current sensor, and thereby, the end point of polishing can be detected on the basis of the detected change in motor current.
The conventional technology, however, does not consider improvement of accuracy of the polishing end point detection.
Namely, the polishing apparatus is sometimes provided with a rotary joint for the purpose of supplying cooling water or the like to the polishing table or the top ring. The rotary joint includes a rotating body which is rotationally driven by a driving unit which drives the polishing table or the top ring, a housing provided around the rotating body, a bearing which is provided between the rotating body and the housing to support rotation of the rotating body, a seal portion which seals between the rotating body and the housing, and the like. The seal portion includes a mechanical seal, an oil seal and the like between the rotating body and the housing. In the seal portion, sliding friction due to the rotating body and the housing arises, and the sliding friction results in driving load of the driving unit which drives the rotating body.
Accordingly, the motor current detected by the current sensor contains not only the driving load due to the polishing friction force but also the driving load due to the sliding friction of the seal portion in the rotary joint. When some cause results in a change in driving load due to the sliding friction of the seal portion, there is a concern that this change is directly reflected to the motor current and causes false detection of the polishing end point.
Therefore, an object of an aspect of the present invention is to improve accuracy of polishing end point detection.